In high-speed optical fiber communication systems, digital data are transmitted in the form of optical pulses propagating in the fiber. An ideal pulse is well localized within a time window and has a well-defined amplitude that stands out distinctly from a low background level. However, noise, chromatic dispersion, and other effects tend to spread the pulses out and to obscure the distinction between pulse and background. These effects can lead to the misinterpretation of high pulse levels (e.g., “ones” in a binary system) as low levels (e.g., as “zeroes” in a binary system) and vice versa. This, in turn, tends to drive up the Bit Error Rate (BER) of the system.
Practitioners have devised regenerators for optical pulses. Ideally, optical energy enters a regenerator as a degraded pulse having a high noise level, a reduced peak amplitude, and expanded width, and exits the regenerator with low background noise and with its original peak amplitude and width restored. Even if they only approximate such ideal behavior, optical regenerators can be advantageous in communication systems for counteracting the degeneration of pulses over long propagation distances.
One particular approach to optical regeneration is described in U.S. Pat. No. 6,141,129, issued on Oct. 31, 2000 to P. V. Mamyshev under the title “Method And Apparatus For All-Optical Data Regeneration.” Central to the Mamyshev regenerator is a nonlinear optical fiber, that is, an optical fiber that can alter the spectral content of a pulse of sufficient amplitude through nonlinear coupling between the fiber material and the electromagnetic field associated with the pulse. As a result of such coupling, stronger portions of a given pulse become spectrally broadened; but the amount of such broadening decreases sharply for weaker portions of the pulse. The spectrally altered pulse is then passed through a filter whose transmission characteristic is offset from the original spectral content of the pulse. We refer to such a filter as an “output filter” of the regenerator. The output filter substantially blocks the weaker portions of the pulse, which were not spectrally broadened, but substantially passes the stronger portions, which contain enhanced spectral content that lies within the passband of the filter. Because only the strongest portion, typically the central portion, of the pulse is passed by the filter, an approximation to the original shape of the pulse is obtained and relatively low background noise is eliminated. If desired, the original amplitude is restored by amplification before the nonlinear fiber, or after it, or both.
Typically, a Mamyshev regenerator includes a dispersion-compensator placed before the nonlinear fiber. The dispersion compensator is an element that has, in effect, a dispersion coefficient opposite in sign to that to which the pulses have been subject while propagating through the system. Such an element is selected, and in some cases can be tuned, to provide a sufficient amount of dispersion to at least approximately cancel the dispersion accumulated during propagation through the system.
We have discovered that in some operating regimes, the performance of the Mamyshev regenerator is very sensitive to the residual dispersion effects that remain impressed upon the optical pulses. However, the magnitude of these effects is not always known in advance. For this reason among others, there is a need for a device to monitor the effectiveness of pulse shaping in a nonlinear optical fiber.